WASHINGTON -- The commander of U.S. and coalition forces in Afghanistan said an air strike on a hospital in the northern city of Kunduz was a mistake, as he reiterated that final approval was made by U.S. officers.

The October 3 attack killed at least 22 people and wounded dozens more at a clinic operated by the medical charity Doctors Without Borders (MSF).

General John Campbell told the Senate Armed Services Committee on October 6 that a U.S. special operations unit was on the ground with Afghan forces, and they had been in contact with the AC-130 gunship that fired on the clinic.

"To be clear, the decision to provide aerial fire was a U.S. decision made within the U.S. chain of command. A hospital was mistakenly struck," he said. "We would never intentionally target a protected medical facility."

"Even though the Afghans requested that support, it still has to go through rigorous U.S. procedure to enable fire to go on the ground," Campbell said, in response to questions from the committee chairman, Senator John McCain.

"We had a special operations unit that was in close vicinity that was talking to the aircraft that delivered those fires," he said.

U.S. Defense Secretary Ashton Carter later underscored the Pentagon's regrets over the incident as he visited Rome on a five-day European tour.

"The U.S. military takes the greatest care in our operations to prevent the loss of innocent life, and when we make mistakes, we own up to them. That's exactly what we're doing right now," Carter said. "The investigation into how this could have happened is continuing, and we are fully supporting NATO and Afghanistan's concurrent investigations."

But Doctors Without Borders dismissed claims the attack was an error and once again asserted that it amounted to a war crime.

"Unfortunately, I am even certain that it was not a mistake," Mego Terzian, president of MSF France, said during a hearing in Paris October 6.

The Swiss-based charity, also known as Medecins Sans Frontieres, has called the incident a “blatant breach of humanitarian law.”

It says the hospital was well known to coalition and Afghan officials, and munitions continued to hit the facility for another 30 minutes even after the charity notified military leadership.

"Until proven otherwise, the events of last Saturday [October 3] amount to an inexcusable violation of this law. We are working on the presumption of a war crime,"said the charity's President Joanne Liu, said in a statement.

Under international humanitarian law, which the United States adheres to, civilian buildings like hospitals or orphanages or religious buildings cannot be targeted by military forces, unless they are being used by fighters to attack opponents.

U.S. Army General John Campbell

Campbell said there were three investigations ongoing: by the Pentagon, NATO, and Afghan military officials. At a briefing later on October 6, White House spokesman Josh Earnest told reporters that senior administration officials have also met with charity officials since the incident.

Amid a dramatic uptick in fighting in Afghanistan this year, Kunduz was overrun by the Taliban on September 28, undermining U.S. officials' confidence in the fighting ability of Afghan security forces, which have reportedly retaken the city as of October 6.

There has been conflicting information about the circumstances of the strike and who ordered it.

Campbell said on October 5 that the attack was requested by Afghan forces who reported being under Taliban fire. The U.S. military had previously claimed that insurgents had been firing at American forces.

Under questioning from senators, the U.S. general also said there needed to be more U.S. troops in Afghanistan after 2016 to contend with potential Taliban offensives and to еnsure the Afghan military is trained and capable of defeating them. The Obama administration, winding down the U.S. presence in Afghanistan, has called for nothing more than a "embassy-based presence" by 2017.

"I do believe that we have to provide our senior leadership options different than the current plan that we're going with," Campbell said.

Antiwar protesters sat in the front row of the Senate hearing with red coloring, depicting blood, on their faces, and carrying signs that read: "Healthcare not warfare," and "Kunduz victims: RIP."

One woman who shouted "Bombing hospitals is a war crime! Stop the bombing now!" was escorted from the room.